Research On Hot Carrier Extraction In CsPbBr₃ Nanocrystals

Due to the excellent structural tolerance and photoelectric properties of perovskite materials,the device efficiency of perovskite solar cells has achieved a rapid breakthrough from 3.8%in 2009 to 25.8%today.however,the further improvement slows down as the efficiency gets closer to the Shockley-Queisser（SQ）limit.The energy loss in the process of rapid hot charge carrier relaxation is one of the main reasons stopping the efficiency of single-junction solar cells bypassing the SQ limit.For the purpose of utilizing the energy of hot carriers,efficient extraction of hot carriers should also be considered.The temperature of photoinduced charge carriers can reflect its distribution of energy states.Thus the temperature of carriers is often used to represent the relaxation process of hot carriers.By accurately obtaining the temperature of hot carriers is crucial for studying their dynamic processes.At present,it is still in debate for the fundamental models to describe the hot carrier temperature in associate with ultrafast optical spectroscopy approaches.Further efforts are needed to explore the relationship between carrier dynamics and underlying physical mechanisms to confirm the reliability of the fitting.In this work,CsPbBr₃perovskite nanocrystals are studied for the merits of intrisically long hot carrier lifetime.Based on the high-quality CsPbBr₃nanocrystals synthesized herein,we establish a global fitting model to reliably obtain the physical parameters in hot carrier dynamics.Furthermore,multiple carrier-accepting molecule systems are studied.The main research contents are as follows:（1）Based on previous study on damping ligand in slowing hot carrier relaxation,we firstly applied 3-aminopropyltriethoxysilane molecule ligands to synthesize uniform CsPbBr₃nanocrystals.By using home-built transient absorption spectroscopy system with high signal-to-noise ratio,initial hot carrier dynamics process was investigated.Subsequently,the changes in bandgap energy caused by the dynamic distribution of hot carriers were intensively studied,the underlying physical mechanisms of transient absorption characteristic signals related to hot carriers were analyzed and the correlation between bandgap changes and physical mechanisms was elucidated.Furthermore,using a band filled global fitting model based on the Fermi-Dirac distribution,the relationship between bandgap shift and the physical parameters of hot carriers extracted from the model was explored,and the reliability of the parameters extracted from the global fitting model was further verified.（2）By temperature controlled synthesis of CsPbBr₃nanocrystals,combined with morphology and optical performance characterization results,high-quality nanocrystals synthesized at 170℃were selected as the basis for hot carrier extraction.By utilizing the conjugatedπ-electron delocalization effect in the aromatic compounds,carrier-accepting molecules with different functional groups were selected and bound to CsPbBr₃nanocrystals through surface ligand exchange process.Combining topographical and optical characterization results,4-hydroxyphenylthiophenol was selected for intensive studies of the hot carrier extraction process.Based on the transient absorption spectroscopy approach and corresponding fitting method as discussed above,it was found the significant hot carrier extraction process occuring at the CsPbBr₃nanocrystal interface,through analysis of the CsPbBr₃bleaching signal and its formation kinetics,as well as the changes of the hot carrier relaxation lifetime.